1. Field of the Invention
The present invention relates generally to cleaning systems, and more specifically to substrate cleaning systems, such as textile cleaning systems, utilizing an organic cleaning solvent and a pressurized fluid solvent.
2. Related Art
A variety of methods and systems are known for cleaning substrates such as textiles, as well as other flexible, precision, delicate, or porous structures that are sensitive to soluble and insoluble contaminants. These known methods and systems typically use water, perchloroethylene, petroleum, and other solvents that are liquid at or substantially near atmospheric pressure and room temperature for cleaning the substrate.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. Recently, however, the desirability of employing these conventional methods and systems has been questioned due to environmental, hygienic, occupational hazard, and waste disposal concerns, among other things. For example, perchloroethylene frequently is used as a solvent to clean delicate substrates, such as textiles, in a process referred to as “dry cleaning.” Some locales require that the use and disposal of this solvent be regulated by environmental agencies, even when only trace amounts of this solvent are to be introduced into waste streams.
Furthermore, there are significant regulatory burdens placed on solvents such as perchloroethylene by agencies such as the EPA, OSHA and DOT. Such regulation results in increased costs to the user, which, in turn, are passed to the ultimate consumer. For example, filters that have been used in conventional perchloroethylene dry cleaning systems must be disposed of in accordance with hazardous waste or other environmental regulations. Certain other solvents used in dry cleaning, such as hydrocarbon solvents, are extremely flammable, resulting in greater occupational hazards to the user and increased costs to control their use.
In addition, textiles that have been cleaned using conventional cleaning methods are typically dried by circulating hot air through the textiles as they are tumbled in a drum. The solvent must have a relatively high vapor pressure and low boiling point to be used effectively in a system utilizing hot air drying. The heat used in drying may permanently set some stains in the textiles. Furthermore, the drying cycle adds significant time to the overall processing time. During the conventional drying process, moisture adsorbed on the textiles fibers is often removed in addition to the solvent. This often results in the development of undesirable static electricity and shrinkage in the garments. Also, the textiles are subject to greater wear due to the need to tumble the textiles in hot air for a relatively long time. Conventional drying methods are inefficient and often leave excess residual solvent in the textiles, particularly in heavy textiles, components constructed of multiple fabric layers, and structural components of garments such as shoulder pads. This may result in unpleasant odors and, in extreme cases, may cause irritation to the skin of the wearer. In addition to being time consuming and of limited efficiency, conventional drying results in significant loss of cleaning solvent in the form of fugitive solvent vapor. The heating required to evaporate combustible solvents in a conventional drying process increases the risk of fire and/or explosions. In many cases, heating the solvent will necessitate explosion-proof components and other expensive safety devices to minimize the risk of fire and explosions. Finally, conventional hot air drying is an energy intensive process that results in relatively high utility costs and accelerated equipment wear.
Traditional cleaning systems may utilize distillation in conjunction with filtration and adsorption to remove soils dissolved and suspended in the cleaning solvent. The filters and adsorptive materials become saturated with solvent, therefore, disposal of some filter waste is regulated by state or federal laws. Solvent evaporation especially during the drying cycle is one of the main sources of solvent loss in conventional systems. Reducing solvent loss improves the environmental and economic aspects of cleaning substrates using cleaning solvents. It is therefore advantageous to provide a method and system for cleaning substrates that utilizes a solvent having less adverse attributes than those solvents currently used and reduces solvent losses.
As an alternative to conventional cleaning solvents, pressurized fluid solvents or densified fluid solvents have been used for cleaning various substrates, wherein densified fluids are widely understood to encompass gases that are pressurized to either subcritical or supercritical conditions so as to achieve a liquid or a supercritical fluid having a density approaching that of a liquid. In particular, some patents have disclosed the use of a solvent such as carbon dioxide that is maintained in a liquid state or either a subcritical or supercritical condition for cleaning such substrates as textiles, as well as other flexible, precision, delicate, or porous structures that are sensitive to soluble and insoluble contaminants.
For example, U.S. Pat. No. 5,279,615 discloses a process for cleaning textiles using densified carbon dioxide in combination with a non-polar cleaning adjunct. The preferred adjuncts are paraffin oils such as mineral oil or petrolatum. These substances are a mixture of alkanes including a portion of which are C16 or higher hydrocarbons. The process uses a heterogeneous cleaning system formed by the combination of the adjunct which is applied to the textile prior to or substantially at the same time as the application of the densified fluid. According to the data disclosed in U.S. Pat. No. 5,279,615, the cleaning adjunct is not as effective at removing soil from fabric as conventional cleaning solvents or as the solvents described for use in the present invention as disclosed below.
U.S. Pat. No. 5,316,591 discloses a process for cleaning substrates using liquid carbon dioxide or other liquefied gases below their critical temperature. The focus of this patent is on the use of any one of a number of means to effect cavitation to enhance the cleaning performance of the liquid carbon dioxide. In all of the disclosed embodiments, densified carbon dioxide is the cleaning medium. This patent does not describe the use of a solvent other than the liquefied gas for cleaning substrates. While the combination of ultrasonic cavitation and liquid carbon dioxide may be well suited to processing complex hardware and substrates containing extremely hazardous contaminants, this process is too costly for the regular cleaning of textile substrates. Furthermore, the use of ultrasonic cavitation is less effective for removing contaminants from textiles than it is for removing contaminants from hard surfaces.
U.S. Pat. No. 5,377,705, issued to Smith et al., discloses a system designed to clean parts utilizing supercritical carbon dioxide and an environmentally friendly co-solvent. Parts to be cleaned are placed in a cleaning vessel along with the co-solvent. After adding super critical carbon dioxide, mechanical agitation is applied via sonication or brushing. Loosened contaminants are then flushed from the cleaning vessel using additional carbon dioxide. Use of this system in the cleaning of textiles is neither suggested nor disclosed. Furthermore, use of this system for the cleaning of textiles would result in redeposition of loosened soil and damage to some fabrics.
U.S. Pat. No. 5,417,768, issued to Smith et al., discloses a process for precision cleaning of a work piece using a multi-solvent system in which one of the solvents is liquid or supercritical carbon dioxide. The process results in minimal mixing of the solvents and incorporates ultrasonic cavitation in such a way as to prevent the ultrasonic transducers from coming in contact with cleaning solvents that could degrade the piezoelectric transducers. Use of this system in the cleaning of textiles is neither suggested nor disclosed. In fact, its use in cleaning textiles would result in redeposition of loosened soil and damage to some fabrics.
U.S. Pat. No. 5,888,250 discloses the use of a binary azeotrope comprised of propylene glycol tertiary butyl ether and water as an environmentally attractive replacement for perchlorethylene in dry cleaning and degreasing processes. While the use of propylene glycol tertiary butyl ether is attractive from an environmental regulatory point of view, its use as disclosed in this invention is in a conventional dry cleaning process using conventional dry cleaning equipment and a conventional evaporative hot air drying cycle. As a result, it has many of the same disadvantages as conventional dry cleaning processes described above.
U.S. Pat. No. 6,200,352 discloses a process for cleaning substrates in a cleaning mixture comprising carbon dioxide, water, surfactant, and organic co-solvent. This process uses carbon dioxide as the primary cleaning media with the other components included to enhance the overall cleaning effectiveness of the process. There is no suggestion of a separate, low pressure cleaning step followed by the use of densified fluid to remove the cleaning solvent. As a result, this process has many of the same cost and cleaning performance disadvantages of other liquid carbon dioxide cleaning processes. Additional patents have been issued to the assignee of U.S. Pat. No. 6,200,352 covering related subject matter. All of these patents disclose processes in which liquid carbon dioxide is the cleaning solvent. Consequently, these processes have the same cost and cleaning performance disadvantages.
Several of the pressurized fluid solvent cleaning methods described in the above patents may lead to recontamination of the substrate and degradation of efficiency because the contaminated solvent is not continuously purified or removed from the system. Furthermore, pressurized fluid solvent alone is not as effective at removing some types of soil as are conventional cleaning solvents. Consequently, pressurized fluid solvent cleaning methods require individual treatment of stains and heavily soiled areas of textiles, which is a labor-intensive process. Furthermore, systems that utilize pressurized fluid solvents for cleaning are more expensive and complex to manufacture and maintain than conventional cleaning systems. Finally, few if any conventional surfactants can be used effectively in pressurized fluid solvents. The surfactants and additives that can be used in pressurized fluid solvent cleaning systems are much more expensive than those used in conventional cleaning systems.
There thus remains a need for an efficient and economic method and system for cleaning substrates that incorporates the benefits of prior systems, and minimizes the difficulties encountered with each. There also remains a need for a method and system in which the hot air drying time is eliminated, or at least reduced, thereby reducing the wear on the substrate and preventing stains from being permanently set on the substrate.